When you can only build a small number of apartments you make the most money building luxury ones. But: (1) If we allow more building we end up with as many l

When you can only build a small number of apartments you make the most money building luxury ones. But: (1) If we allow more building we end up with as many luxury apartments as there's demand for and developers switch to building more affordable ones. (2) When a rich person moves from wherever they're living now into a luxury apartment that opens up their previous apartment somewhere else.